The present invention relates to a ferroelectric memory device.
As a ferroelectric memory device, an active ferroelectric memory device including a 1T/1C cell in which one transistor and one capacitor (ferroelectric) are disposed in each memory cell, or including a 2T/2C cell in which a reference cell is further disposed in each memory cell, has been known.
However, since the active ferroelectric memory device has a large memory area in comparison with a flash memory or EEPROM, which is known as a nonvolatile memory device in which a memory cell is formed by one element, the capacity cannot be increased.
A ferroelectric memory device in which each memory cell is formed by one ferroelectric capacitor is known (Japanese Patent Application Laid-open No. 9-116107). Japanese Patent Application Laid-open No. 9-116107 discloses hierarchization of a bitline. Specifically, a plurality of sub-bitlines through a plurality of connection means are provided for one main-bitline. When one of the connection means is turned on, one main-bitline can be connected with one sub-bitline selected by turning on the connection means. This prevents voltage from being applied to the unselected memory cells connected with other sub-bitlines, whereby the number of disturbances applied to the unselected memory cells can be limited.
However, the sub-bitline connected with the connection means that has been turned off floats. The interconnect potential changes if noise is applied from the outside, whereby data stored in the ferroelectric capacitor connected with the sub-bitline may be destroyed.
Japanese Patent Application Laid-open No. 7-235648 discloses a ferroelectric memory device which includes a plurality of sectors divided in units of sub-bitlines and in which each sector is further divided into a plurality of sub-sectors. The sector selected from among the plurality of sectors (selected sector) is divided into a selected sub-sector and an unselected sub-sector. In the selected sector, the sub-bitlines do not float in both the selected sub-sector and the unselected sub-sector.
However, the potential of the sub-bitline is in a floating state in all the unselected sub-sectors in the unselected sector.